building docs implicitly depends on building tests

Since commit e1744603 building docs for glib fails if tests are disabled.

To be exact, things fail at the configure stage.

This has been reported for example in Gentoo bug 777636.

Simply passing 'gio/tests/gdbus-object-manager-example' as subdir in top level meson file doesn't suffice, as test_c_args and gdbus_example_objectmanager_sources are defined in an earlier meson file.

Adding those those at top level seems to work, but seems a messy way to solve this.